Feature: TSV-reports sorted by abundance

[samuell]

I noted that the *-rel-abundance.tsv and *-read-assignment-distributions.tsv are not sorted by abundance.

This can of course be done in downstream tools, but downstream analysis would be greatly simplified if they were sorted by abundance from the start, especially for plotting the *-read-assignment-distrubutions.tsv file that will now need to check the abundance in the *-rel-abundance.tsv file to sort it correctly.